1. Field of the Invention
This invention relates to laser systems, and more particularly, to laser systems for use in optical storage devices.
2. Description of the Prior Art
Semiconductor diode lasers are used in optical data storage systems. The gallium-aluminum-arsenide (GaAlAs) diode laser is one example and it generates light in the near infrared range (750-880 nanometers wavelength). The light from the laser is focused onto a spot on the optical disk in order to record each bit of data. The diameter of the spot is equal to approximately .lambda./2(N.A.), where .lambda. is the wavelength of the light and (N.A.) is the numerical aperture of the lens which focuses the laser light. For a GaAlAs laser of 830 nm wavelength and a lens with a (N.A.) of approximately 0.5, the resulting spot size is 860 nanometers in diameter.
It is apparent that if the wavelength of the laser light can be cut in half, the diameter of the spot size will also be cut in half and the overall density of the optical disk will be quadrupled. Unfortunately, laser diodes which produce light in the blue range (430 nm in wavelength) are not yet available. Research in this area has concentrated on ways to convert the infrared light from the laser diode into blue light.
One technique to convert light to a higher frequency is known as second harmonic generation (SHG). Light is passed through a nonlinear crystal, such as potassium niobate (KNbO.sub.3) and the second harmonic light (light at twice the frequency of the fundamental light) is generated. This SHG technique is discussed in the articles by M. K. Chun, et al., Applied Physics Letters, Sept. 26,1988, Vol. 53, No. 13, p. 1170; P. Gunter, et al., Applied Physics Letters, Sept. 15, 1979, Vol. 35, No. 6, p. 461; and by P. Gunter, et al., Optics Communications, Dec. 1, 1983, Vol. 48, No. 3, p. 215. However, the input power available for the diode lasers is low, unless additional optical enhancement techniques are used.
One way to increase the efficiency of the SHG scheme is to place an optical resonator around the nonlinear crystal. The light is reflected back and forth through the crystal inside the resonator in order to generate a substantial amount of the blue light. This technique is described by W. J. Kozlovsky, et al., IEEE Journal of Quantum Electronics, June 1988, Vol. 24, No. 6, p. 913; W. J. Kozlovsky, et al., Optics Letters, December 1987, Vol. 12, No. 12, p. 1014; A. Ashkin, et al., "Resonant Optical Second Harmonic Generation and Mixing," IEEE J. Quantum Electronics, QE-2, 109-123, (1966); and by P. W. Smith, Proceedings of the IEEE, April 1972, Vol. 60, No. 4, p. 422. The disadvantage of this scheme is that the frequency of the laser must be precisely tuned to the resonant frequency of the resonator (otherwise known as a passive cavity) and must somehow be stabilized so that it remains locked to the resonator cavity resonance at all times. The laser frequency must be stable to within a fraction of the width of the resonance of the passive cavity.
For example, assume that there is a nonlinear resonator with an effective length (including refractive-index contribution of the nonlinear crystal) of 1.5 centimeters, then the resonator cavity mode spacing is equivalent to 10 gigahertz. A finesse of approximately 100 is needed to build up high circulating power and the linewidth of the resonance will be approximately 100 megahertz. For efficient frequency second harmonic generation, the laser must be frequency locked to within an accuracy of less than approximately 20 megahertz, which is comparable to the intrinsic linewidth of diode lasers.
The laser may be actively locked to the resonant frequency by means of an electronic detection and feedback circuit. This greatly increases the complexity of the system. The laser may alternatively be passively locked to the resonant frequency. If light at the resonant frequency is directed back into the laser, the laser can stabilize its frequency to that of the resonant frequency. See the article by B. Dahmani, et al., Optics Letters, November 1987, Vol. 12, No. 11, p. 376.
A paper by G. J. Dixon, et al., Optics Letters, July 15, 1989, Vol. 14, No. 14, p. 731, teaches a SHG system using passive laser locking. The Dixon device uses an angled mirror at the exiting end of the nonlinear crystal resonator to separate a small portion of the fundamental frequency light from the second harmonic frequency light. This small portion of the fundamental frequency light is then reflected back around the nonlinear crystal resonator and into the laser via a 1/2 wave plate, a second mirror, a polarizing beam splitter, and a magneto-optical isolator. The laser is thereby effectively locked at the resonant frequency of the nonlinear crystal resonator. U.S. Pat. No. 4,884,276 by G. J. Dixon, et al. teaches another SHG system which uses optical feedback. Both of these systems require that the feedback light be precisely controlled so that it is in phase with the laser light field. What is needed is a laser SHG system with optical feedback having a minimum number of optical parts.